A burner for gaseous fuels, as used in particular in a gas turbine installation, is known from example from DE 42 12 810 A1. According to this, combustion air is fed through an annular air duct system and fuel is fed through a further annular duct system for combustion. A high-calorie fuel (natural gas or fuel oil) is thereby injected from the fuel duct into the air duct, either directly or from helical blades configured as hollow blades.
The most homogenous mixture possible of fuel and air should therefore be obtained, in order to achieve combustion with low levels of nitrogen oxide. For environmental protection reasons and because of corresponding legal provisions governing pollutant emissions, the lowest possible level of nitrogen oxide production is an important combustion requirement, in particular for combustion in the gas turbine installation of a power plant. The formation of nitrogen oxides increases exponentially with flame temperature during combustion. If the fuel/air mixture is non-homogenous, a certain distribution of flame temperatures results in the combustion area. The maximum temperatures of such a distribution then determine the quantity of nitrogen oxides formed according to the cited relationship between nitrogen oxide formation and flame temperature. Combustion of a homogenous fuel/air mixture thus achieves a lower nitrogen oxide emission for the same mean flame temperature than combustion of a non-homogenous mixture. The burner design in the publication cited above achieves a spatially good air/fuel mixture.
Compared with the conventional gas turbine fuels, natural gas and crude oil, which essentially comprise hydrocarbon compounds, the combustible components of synthesis gas are essentially carbon monoxide and hydrogen. For the optional operation of a gas turbine with synthesis gas from a gasification facility and a secondary or substitute fuel, the burner in the combustion chamber assigned to the gas turbine must be designed as a twin or multi-fuel burner, which can be fed both the synthesis gas and the secondary fuel, e.g. natural gas or fuel oil as required. The respective fuel is hereby fed to the combustion zone via a fuel passage in the burner.
Depending on the gasification method and the overall installation design, the calorific value of the synthesis gas is around five to ten times less than the calorific value of natural gas. The main components in addition to CO and H2 are inert elements such as nitrogen and/or steam and in some instances also carbon dioxide. Its low calorific value means that large flow volumes of combustion gas have to be fed through the burner to the combustion chamber. This means that one or more separate fuel passages have to be provided for the combustion of low-calorie fuels, such as synthesis gas. Such a multi-passage burner, which is also suitable for synthesis gas operation, is disclosed for example in EP 1 227 920 A1.
As well as the stoichiometric combustion temperature of the synthesis gas, the quality of the synthesis gas/air mixture in front of the flame is an important factor influencing the prevention of temperature peaks and thus impacting on the minimization of thermal nitrogen oxide formation.
As far as the increasingly stringent requirements relating to nitrogen oxide emissions are concerned, premix combustion is of increasing significance even for the combustion of low-calorie gases.